linux-kernel - Re: [RESEND PATCH 0/2] iommu/iova: optimize the iova rcache

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Date:   Fri, 11 Aug 2023 14:25:31 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Zhang Zekun <zhangzekun11@...wei.com>, joro@...tes.org,
        will@...nel.org
Cc:     iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
        baolu.lu@...ux.intel.com, robh@...nel.org, nicolinc@...dia.com,
        john.g.garry@...cle.com, kevin.tian@...el.com
Subject: Re: [RESEND PATCH 0/2] iommu/iova: optimize the iova rcache

On 2023-08-11 14:02, Zhang Zekun wrote:
> The number of iova_cpu_rcache can grow with the number of cpus in
> iova_rcache, but the size of rcache->depot will not. The deeper of
> rcache->depot can help iova_rcache cache more iovas, and can help
> iova_rcache better dealing with senarios in which drivers allocating
> and free iovas on different cpu cores. We only let the size of rcache->depot
> to grow with the number of cpus which is larger than 32 to avoid potential
> performance decrease on machines which don't have much cpus.

Oh, that reminds me I had started working on this again just before I 
was off, only my plan was to get rid of the depot allocation 
altogether[1]. However I wanted to finish the second patch on that 
branch (to reclaim wasted memory from the depot if the workload changes) 
before posting anything - I was feeling really pleased that I'd made it 
work until I realised it was leaking all the actual IOVAs... :(

Thanks,
Robin.

[1] 
https://gitlab.arm.com/linux-arm/linux-rm/-/commit/2f37ebe93eb282b534bf9e0fd4adc66cfe4b6693

> 
> Also, it is unsafe to directly free cpu rcache magazines in free_iova_rcaches,
> add check before freeing it.
> 
> Zhang Zekun (2):
>    iommu/iova: Add check for cpu_rcache in free_iova_rcaches
>    iommu/iova: allocate iova_rcache->depot dynamicly
> 
>   drivers/iommu/iova.c | 36 +++++++++++++++++++++++++++++++-----
>   1 file changed, 31 insertions(+), 5 deletions(-)
>